1. Field of the Invention
The invention relates to a vibrating type driving device for obtaining a driving force by press contacting a moving member against a vibration member.
2. Related Background Art
In a vibrating type driving device (vibrating type motor), a moving member is brought into pressure contact with a vibration member made of an elastic member to which an electromechanical energy converting element is joined; an AC voltage is applied to the converting element, and a progressive vibration wave thereby is generated in the vibration member, thereby frictionally driving the moving member.
FIG. 7 shows a conventional vibrating type motor. A ring-shaped stator (vibration member) 102 fixed to a base 101 is constructed in a manner such that an electromechanical energy converting element 122 to which a current is supplied through a connector 110 and a flexible board 111 is joined to the lower surface of an elastic member 121 and a frictional member 123 is adhered onto the upper surface of the elastic member 121. An outer peripheral portion of a pressurizing spring 106 is attached onto the upper surface of a rotor (moving member) 103 through a rubber plate 107. An inner peripheral portion of the pressurizing spring 106 is attached to a disk 105 which is shrink fitted to an output shaft 104.
The output shaft 104 is rotatably supported by a pair of roller bearings 181 and 182 each having an outer ring fixed to the base 101 and an inner ring fitted to the outer circumference of the output shaft 104. The disk 105 is in contact with the inner ring of the rolling bearing 182. On the other hand, the inner ring of the rolling bearing 181 is brought into engagement with a snap ring 109 attached to a groove of the output shaft 104 in a state in which the output shaft 104 is pressed onto the stator 102 side together with the disk 105 and the inner ring of the roller bearing 182 by only a displacement amount of the pressurizing spring 106, thereby bringing the rotor 103 into pressure contact with the stator 102 with a proper force.
As shown in FIG. 6, consequently, in the bearing 182, a preload force acting in the same direction as that of a pressure of the pressurizing spring 106 is applied to the inner ring by the disk 105, so that a rattle in the radial direction in the bearing 182 is eliminated. On the other hand, in the bearing 181, a preload force acting in the direction opposite to that of the pressure of the pressurizing spring 106 is applied to the inner ring by the snap ring 109, so that a rattle in the radial direction in the bearing 181 is eliminated. Since the rattle in the radial direction of each of the bearings 181 and 182 is eliminated, a shake in the radial direction of the output shaft 104 is also suppressed.
Now, assuming that a reactive force of pressure of the pressurizing spring 106 is labeled as F and a preload force of the bearing 181 is labeled P1 and a preload force of the bearing 182 is called P2, the relation
F=P1-P2
is satisfied among those three forces.
As will be understood from the above relation, since the bearing 181 receives the sum of the reactive force of pressure and the preload force of the bearing 182 as a preload force, such a preload force is much larger than the preload force of the bearing 182.
Usually, a fatigue life of the bearing is inversely proportional to the cube of the bearing load. Therefore, a fatigue life of the bearing 181 which receives a large preload force is much shorter than that of the bearing 182. Further, since the vibrating type motor is often used at a low speed and it is difficult to form an oil film between the rolling member of the bearing and a raceway surface, it is necessary to set a load of a rolling member of the bearing to be smaller than the ordinary load.
A torque which can be generated by a motor depends on the maximum frictional force between the stator and the rotor. Since the frictional force is determined by a coefficient of friction between the rotor and the stator, and a pressure applied therebetween, it is effective to increase the pressure in order to raise the maximum torque.
In the conventional motor, however, since the bearing 181 bears all of the pressure, in order to increase the pressure without reducing the bearing life, a bearing having a larger load rating has to be used. This results in an increase in size and cost of the bearing.
In recent years, in the fields of OA equipment and FA equipment, high precision is demanded in the positioning of a driving mechanism, the speed control, and the like. A general way to reduce a rotational output of a pulse motor or the like is to use a speed reducing mechanism such as gear, belt, or the like and to perform driving at high resolution and high torque. When the speed reducing mechanism uses gear, however, transfer precision may deteriorate due to a tooth shape error, a pitch circle error, or the like. In order to raise the degree of precision, it is necessary to raise the gear grade, perform a precision grinding, or the like, which results in high costs. Further, since controllability deteriorates due to nonlinearity caused by backlash, it is necessary to provide a countermeasure, such as a non-backlash gear or the like. When the speed reducing mechanism uses a belt, the controllability also deteriorates because of a reduction in transfer precision due to eccentricity, roundness, or the like of a pulley, expansion and contraction of the belt, and reduction of the transfer rigidity due to a bending vibration.
On the other hand, a "direct drive" method, in which a motor shaft is directly attached to a driven member and is driven using a motor which can generate a low speed and a high torque without using a speed reducing mechanism such as a gear, belt, or the like, is an effective drive means. In this "direct drive" method, reduction of the precision using the above transfer mechanism, backlash, and reduction of the rigidity in the transfer system can be prevented and the motor shaft can be driven at high precision.
Since a vibration wave motor can stably generate torque at a low speed, it is a motor suitable for the "direct drive" method. The vibration wave motor is constructed in a manner such that an electromechanical energy converting element (piezoelectric element, magnetostrictive element, or the like) is joined to one side of a vibration member made of an elastic material, an AC voltage is applied to such a piezoelectric element, and a progressive vibration wave is generated in the vibration member, thereby frictionally driving the moving member that is in contact with the vibration member with a pressure. By combining angle detecting means of a high resolution to such a motor, the motor shaft can be driven at high precision, high rigidity, and high resolution.
FIGS. 15A and 15B show conventional examples of a roller driving device which is used to convey a sheet.
One end of a roller is rotatably supported at a casing of the device by a ball bearing and another end is fixed to an output shaft of a motor fixed to the device. With such a structure, the transfer error occurring in the conventional case of reducing the speed by using a gear or belt driving can be eliminated.
In the case of performing a gear or belt driving method, even if a slight error occurs in an attaching precision between the member to be driven and the transfer mechanism, it can be absorbed by the transfer mechanism. For example, although a change in distance between the shafts occurring when the eccentricity of the gear causes deterioration of the transfer precision, since the displacement of the gear is absorbed between the gears, a surplus load that is caused by an error of each transfer member is not applied to the driven member or the motor. In the case of belt driving as well, since each part precision and an attaching error are converted to a linear velocity of the belt or are absorbed by the extension and contraction of the belt, the load to be applied to the motor is small.
However, in the case of performing a "direct drive" method or in the case where at least one end of the rotary shaft of the driven member is supported by a bearing, since the motor shaft is supported at three points of two bearings in the motor and one bearing of the driven member, there is no location where an inclination of a coupling portion of the motor shaft and the driven member or an axial deviation is absorbed. Therefore, a large load is given to each bearing, the bearing life is reduced, precision due to increase in radial oscillation deteriorates, and the torque of the motor is reduced due to the load.
For example, FIG. 15A shows a state in which a deep groove ball bearing 5' in which a proper preload is applied is arranged at an edge of a roller 2' serving as a member to be driven on the side opposite to the side where a motor 11 is coupled.
A construction of the motor 1' will now be described. In a stator 12' fixed to a base 11' with screws, a piezoelectric element is fixedly attached to the back surface of a vibration member using an elastic material such as metal or the like and a frictional material is adhered to another surface thereof. A rotor 13' is brought into pressure contact with the stator 12' by a pressurizing spring 16' whose bore is fixed to a disk 15' which is shrink fitted to a rotary shaft 14'. The rotary shaft of the rotor 13' is rotatably supported by a pair of deep groove ball bearings 17'-1 and 17'-2 in each of which an outer ring is fixed to a base. The rotor 13' is supported at an inner ring of the bearing 17'-1 by a snap ring 18' attached to a groove of the rotary shaft in a state where the rotary shaft is pushed onto the stator side by only a pressurizing displacement amount of a pressurizing spring for making the rotor come into pressure contact with the stator with a proper force. The bearing 17'-2 eliminates a rattle by applying a proper preload force to the inner ring by the disk 15' and suppresses a radial oscillation of the rotary shaft, thereby guaranteeing a precision of a built-in encoder 6'.
By supplying a driving voltage to the piezoelectric element through a connector flexible board 19', a progressive vibration wave is generated in the stator, and the rotor that is in pressure contact with the stator is driven, thereby taking out the rotation from the rotary shaft.
In the coupling between the motor and the roller, the rotary shaft 14' is inserted with a light pressure into a hole 24' formed in the roller and is fixed by a set screw 25' set from the lateral direction.
In this example, since a preload force is applied to all of the three bearings, the rattle of each bearing is eliminated and a high rigidity can be provided in both the radial and thrust directions.
When an inclination between the roller and the motor shaft occurs in the coupling portion of the roller and the motor, the displacement at the time of rotation has to be absorbed by a deformation of any one of the three bearings or any one of the coupling portion, the motor shaft, and the roller.
This deformation becomes a large load for the driving by the motor, causing deterioration of a stop precision or a speed controllability. Further, since a large radial load occurs in each bearing, the bearing life deteriorates.
When the motor shaft swings and rotates, the pressurizing spring also swings and rotates, so that the pressure with which the rotor 13' is brought into contact with the stator 12' also changes together with the rotation. Thus, a constant torque is not generated.
FIG. 15B shows a state where the motor is fixed to a casing of the driven device using a mount 7' made of rubber in consideration of the above point. When a bending force is applied to the motor, the mount is deformed and absorbs the displacement, so that a surplus load is not applied to the bearings of the motor and the roller. In this case, however, the mount is also deformed in the torsional direction (roller rotating direction) and is rotated and deformed together with the motor casing. In a motor having an angle detecting mechanism (rotary encoder) as shown by the encoder 6', although a rotational displacement due to the torsional deformation of the mount is not detected from the encoder, since an angular displacement occurs in the roller, the encoder cannot accurately detect the angular displacement of the roller.
Further, since a torsional rigidity of the motor casing is low for the load torque, a response speed decreases and the controllability deteriorates.
Although the above example has been described with respect to the "direct drive" method, even in the case where the device has speed reducing and transfer means, a similar problem also occurs in the case of raising the transfer precision and the rigidity by eliminating backlash, using a steel belt having high rigidity, or the like. Therefore, the above problem is not limited to the "direct drive" method.
As mentioned above, in order to improve the precision, it is necessary to fix the motor casing with high rigidity for torsion. Moreover, in the case of using the "direct drive" method or a transfer system of high rigidity, it is necessary that the motor casing is flexibly supported for any error that is caused by a working tolerance or an assembling tolerance.